Black silicon is a revolutionarily new material structure of the electronic industry, usually referring to silicon face or silicon-based film with a high absorptivity. Compared with the structures of common silicon materials, it has a fairly strong light absorption property. If it is applied to optical sensors or solar cells, then their photosensitivity would be 100 times better and therefore the conversion efficiency of solar cells can be improved significantly.
The fabrication of black silicon began with the experiment done by Professor Eric Mazur at Harvard University in 1999, during which he and his graduate students put silicon wafers into a vacuum environment filled up with halogen gas, and then scanned them using an ultra-intense and ultra-short-time laser beam (femtosecond laser). The surfaces of the scanned silicon wafers turned black, but not that caused by scorch. The researchers looked through the electronic microscope and observed that a forest-like peg structure was formed on such wafer surface, which facilitated reduction of ray reflection and improvement of light absorption property.
Later, the Professor Eric Mazur and his team also used a common gas of the semiconductor industry—sulfur hexafluoride (SF6), specifically, they put common silicons into an enclosed space filled up with SF6 and then bombarded them with the strong pulse of a femtosecond laser. After 500 times of pulse scanning, the silicon wafers presented black through naked eye. With the help of electronic microscope, the researchers observed that the wafer surface was an extremely thin peg structure and this procedure roughened the wafer surface. However, fabricating black silicon using femtosecond laser involves complex process, complicated procedural controls, costly equipment and inconvenient maintenance, not facilitating volume production.
The Chinese Patent Specification CN 101734611 A (publication date: Jun. 16, 2010) disclosed a black silicon fabrication method based on maskless deep reactive ion etching, which includes the following steps: let the equipment required for said black silicon fabrication method undergo initialization and plasma stabilization in order to allow for the plasmas' glow discharge; control the process parameters of said black silicon fabrication method based on deep reactive ion etching, and process silicon wafers alternately by etching and passivation; wherein said parameters include plasma gas flow, etching plate power, passivating plate power, coil power and etching, passivation period, and temperature. Although this method for fabricating black silicon is more efficient and cost-effective compared with that using a femtosecond laser, it has some disadvantages such as complex process, complicated procedural controls and low efficiency as it requires silicon wafers to be alternately etched and passivated, that is, repeatedly implanting etching gas and passivation gas and also adjusting corresponding flow, power, time and environmental parameters.